Skip-cycle operation of a four cycle engine, with selected cylinders deactivated in accordance with a predetermined schedule as a function of the change in load, provides opportunities for fuel economy and control of emissions. An engine with a camless valvetrain offers exceptional opportunities for implementation of skip-cycle strategies at part-load.
U.S. Pat. No. 5,255,641, Variable Engine Valve Control System, Schechter, assigned to the assignee of this invention, and which is hereby incorporated by reference into this specification, is an example of a camless valvetrain in which the valve timing and lift, as well as the event duration are controlled by solenoid valves. In such a system, the action of the valves in each engine cylinder can be completely deactivated and then reactivated again within as little as one engine cycle of two revolutions. When coupled with a comparable ability to activate and deactivate the individual fuel injectors and spark plugs, this permits running the engine with a variety of skip-cycle strategies.
The variety of firing orders that can be selected tinder different engine operating conditions is enhanced by the fact that in an engine with a camless valvetrain, any upward stroke of the piston can operate as a compression stroke or as an exhaust stroke, and any downward stroke of the piston can alternate as an intake stroke or as an expansion stroke. The ability to isolate each cylinder so that the intake and exhaust valves can be individually controlled to disable the cylinder results in less energy dissipation within that cylinder.
If the intake valve is first closed after fuel delivery is terminated, followed by closing the exhaust valve, the cylinders are essentially purged of exhaust gases. Therefore, as long as the cylinder is disabled, the downward stroke of the piston creates merely a vacuum and the upward stroke will return the cylinder to essentially atmospheric pressure, thus permitting unthrottled operation of the engine with less friction in the deactivated cylinders.
This individual control of the intake and exhaust valves not only permits varying skip-cycle strategies as a function of change in load, but also it permits control of the timing of the closure of the intake valve during load conditions when each firing cylinder operates at part load to maintain unthrottled operation at all engine loads.
The invention utilizes the essentially instantaneous action of a camless valvetrain together with other strategies including those described above to provide a four cycle engine with a variety of skip-cycle operations that improve fuel economy and engine efficiency while reducing emission output.